1. Field of the Invention
This invention relates to a radiation image detecting system, and more particularly to an improvement of quality of a radiation image reproduced on the basis of image data output from a radiation image detecting system.
2. Description of the Related Art
There has been known a radiation image information read-out system using radiographic film or a stimulable phosphor sheet in medical radiography.
Recently there has been proposed a radiation image detecting system employing a solid radiation detector including a semiconductor as a major part which detects radiations and converts the intensity of radiations to an electric signal. Though various types of solid radiation detectors have been proposed, the following solid radiation detectors are representative.
That is, a solid radiation detector comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of photoelectric conversion elements (each forming a picture element) on an insulating substrate and a phosphor layer (scintillator) which is formed on the two-dimensional image reader and generates visible light bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca photo-conversion type solid radiation detectorxe2x80x9d, hereinbelow.
A solid radiation detector comprising a two-dimensional image reader formed by two-dimensionally forming a plurality of charge collection electrodes (each forming a picture element) on an insulating substrate and a radiation-conductive material layer which is formed on the two-dimensional image reader and generates electric charges bearing thereon image information when exposed to radiations bearing thereon image information. The solid radiation detector of this type will be referred to as xe2x80x9ca direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The photo-conversion type solid radiation detectors are disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164057, PCT International Publication No. WO92/06501, xe2x80x9cSignal, noise, and read out considerations in the development of amorphous silicon photodiode arrays for radiography and diagnostic x-ray imagingxe2x80x9d, L. E. Antonuk et. al., University of Michigan, R. A. street Xerox, PARC, SPIE vol. 1443, xe2x80x9cMedical Imaging Vxe2x80x9d, Image Physics (1991), pp. 108-119, and the like.
As the direct conversion type solid radiation detector, the following have been proposed.
1) A solid radiation detector which is about ten times as large as normal solid radiation detectors in thickness as measured in the direction in which radiations are transmitted through the solid radiation detector. See xe2x80x9cMATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORSxe2x80x9d, Lawrence Berkeley Laboratory. University of California, Berkeley, Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304.
2) Those comprising a plurality of solid radiation detectors laminated in the direction in which radiations are transmitted with metal plates intervening therebetween. See xe2x80x9cMetal/Amorphous Silicon Multilayer Radiation Detectorsxe2x80x9d, IEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36. NO.2 APRIL 1989.
3) Solid radiation detectors using CdTe and the like disclosed in Japanese Unexamined Patent Publication No. 1(1989)-216290.
This applicant has proposed an improvement on the direct conversion type solid radiation detector as disclosed in Japanese Patent Application 9(1997)-222114. The solid radiation detector will be referred to as xe2x80x9can improved direct conversion type solid radiation detectorxe2x80x9d, hereinbelow.
The improved direct conversion type solid radiation detector comprises a first conductive layer which is transparent to recording radiations, a recording photoconductive layer which exhibits photoconductivity upon exposure to the recording radiations passing through the first conductive layer, a charge transfer layer which acts substantially as an insulator to electric charges of the same polarity as that in which the first conductive layer is charged and as a conductor to electric charges reverse to that in which the first conductive layer is charged, a read-out photoconductive layer which exhibits photoconductivity upon exposure to read-out electromagnetic waves, and a second conductive layer which is transparent the read-out electromagnetic waves. These layers are superposed one on another in this order and latent image charges are collected on the interface between the recording photoconductive layer and the charge transfer layer.
As a system for reading out the latent image charges in the improved direct conversion type solid radiation detector, there may be employed a read-out system where the read-out electrode (the second conductive layer) is made like a flat plate and the latent image charges are read out by scanning the read-out electrode with a read-out light spot such as a laser beam, or a read-out system where the read-out electrode is made like a stripe electrode (comb tooth electrode) and the latent image charges are read out by scanning the stripe electrode with a line light beam, extending in a direction perpendicular to the longitudinal direction of the stripe electrode, in the longitudinal direction of the stripe electrode.
In a radiation image detecting system using such a solid radiation detector, a radiation image which is poor in quality can be reproduced if the radiation image is reproduced on the basis of digital image data obtained by digitizing image data as it is output from the solid radiation detector. This is because the level of the image signal components output from the individual solid radiation detecting elements varies according to the radiographing conditions and/or the individual difference of the solid radiation detector, and accordingly if the radiation image is reproduced on the basis of digital image data obtained by digitizing image data as it is output from the solid radiation detector, the density and the contrast of the image cannot be always good. In this specification, the term xe2x80x9csolid radiation detecting elementxe2x80x9d means an element which includes said photoelectric conversion element or the charge collection electrode as a major part and forms a picture element.
Further since the amount of a latent image charge which each solid radiation detecting element can store is limited, the radiation image detecting system using the solid radiation detector is lower in signal saturation level and narrower in dynamic range as compared with the conventional radiation image information read-out system using radiographic film or a stimulable phosphor sheet. Accordingly, even in the common working range, some picture elements can be saturated, the radiation image taken by the radiation image detecting system using the solid radiation detector is lower in quality than that taken by the conventional radiation image information read-out system using radiographic film or the like.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image detecting system using a solid radiation detector which can output image data which is improved in image quality.
In accordance with the present invention, there is provided a radiation image detecting system comprising
a radiation detecting means comprising a plurality of solid radiation detecting elements which are two-dimensionally arranged and convert radiations bearing thereon image information to an image signal,
an A/D convertor which converts the image signal as output from the radiation detecting means to a digital image signal,
a standardization characteristic determining means which analyzes the digital image signal and determines a characteristic of standardization processing which is to be carried out on the digital image signal, and
a standardization processing means which carries out standardization processing on the digital image signal according to the characteristic of standardization processing determined by the standardization characteristic determining means.
The standardization processing is signal processing for processing the digital image signal to conform to a proper input signal range of an image reproduction system or the like connected to the radiation image detecting system so that the density and/or the contrast of the reproduced image becomes proper. For example, by the standardization processing, the digital image signal is converted to an image signal suitable for the image reproduction system or the like in such a manner that maximum and minimum signal levels of the digital image signal in a desired image information range respectively conform to maximum and minimum values in a proper density range of an output visible image.
It is preferred that the radiation image detecting system be provided with a desired signal section determining means which determines a section of the digital image signal bearing thereon a desired section of the image information (this section of the digital image signal will be referred to as xe2x80x9cthe desired digital image sectionxe2x80x9d, hereinbelow) and that the standardization characteristic determining means analyzes the desired digital image signal section determined by the desired signal section determining means and determines the characteristic of standardization processing.
The desired digital image signal section may be determined in any manner. For example, image information in an irradiation field of the radiations may be taken as the desired section of the image information. In this case, as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 61(1986)-39039, 61(1986)-170178, and 63(1988)-259538, the irradiation field may be determined by various known irradiation field recognizing processing such as those in which dynamic edge extraction processing such as using Snakes algorithm is employed, those in which edge extraction processing such as using Hough transformation is employed or those in which prospective edge points which are considered to be on the edge of the irradiation filed are detected and the area surrounded by a line joining the prospective edge points is recognized to be the irradiation field. Further image information only on the object may be taken as the desired section of the image information as disclosed in Japanese Unexamined Patent Publication No. 4(1992)-11242. In this case, the peripheral edge of the image of the object is detected and the section of the image signal representing the area inside the detected peripheral edge may be taken as the desired digital image signal section. Further, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 1(1989)-50171, image information only on a part of the object, e.g., cervical vertebrae and soft part, may be taken as the desired section of the image information. In this case, the section of the digital image signal representing the cervical vertebrae and the soft part is detected and the desired digital image signal section is determined on the basis of the detection.
It is preferred that the standardization characteristic determining means determines the characteristic of the standardization processing taking into account saturation properties of the solid radiation detecting elements, i.e., on the basis of information on saturation of the solid radiation detecting elements including the proportion of the number of the saturated solid radiation detecting elements to the total number of the solid radiation detecting elements, the position of the saturated solid radiation detecting elements in the image, or the like.
It is preferred that the standardization processing includes processing for reducing the bit numbers.
The radiation image detecting system of the present invention may be provided with a memory which stores standardized digital image signal.
It is preferred that the radiation image detecting system of the present invention be provided with an image display means which displays an image on the basis of the standardized digital image signal, and a standardization characteristic changing means for changing the characteristic of the standardization processing.
It is further preferred that the radiation image detecting system of the present invention be provided with an image signal output means which, when the characteristic of the standardization processing is not changed within a predetermined time after an image is displayed by the image display means, automatically outputs the digital image signal on the basis of which the image is displayed.
It is further preferred that the radiation image detecting system of the present invention be provided with an alarm means which gives the alarm when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics.
It is further preferred that the radiation image detecting system of the present invention be provided with a correction information output means which, when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics, outputs correction information for correcting the radiographing conditions to conditions which can present a digital image signal the characteristic of the standardization processing for which may be one of the predetermined characteristics.
It is otherwise preferred that the radiation image detecting system of the present invention be provided with a radiographing condition correcting means which, when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics, changes the radiographing conditions on the basis of one of the predetermined characteristics to conditions which can present a digital image signal the characteristic of the standardization processing for which may be said one of the predetermined characteristics.
In the radiation image detecting system of the present invention, since the digital image signal is subjected to standardization processing the characteristic of which is determined by analyzing the digital image signal, the digital image signal is made to conform to a proper input signal range of an image reproduction system or the like connected to the radiation image detecting system without affected by the radiographing conditions and/or the individual difference of the solid radiation detector used, and accordingly the density and/or the contrast of the reproduced image becomes proper.
When the standardization characteristic determining means analyzes the desired digital image signal section determined by the desired signal section determining means and determines the characteristic of standardization processing, the standardization characteristic determining means can determine more suitable characteristic of the standardization processing on the basis of only the digital image signal section representing effective section of the radiation image information.
Further when the standardization characteristic determining means determines the characteristic of the standardization processing taking into account saturation properties of the solid radiation detecting elements, the characteristic of the standardization processing can be determined without affected by saturation properties of the solid radiation detecting elements.
Further when processing for reducing the bit numbers is carried out on the digital image signal together with the standardization processing, the amount of data handled by the image reproducing system connected to the radiation image detecting system is reduced and the image reproducing system may be on a small scale, whereby the image reproducing system can be compact in size.
When the radiation image detecting system is provided with a memory which stores standardized digital image signal, the standardized digital image signal can be read out and an image can be reproduced on the basis of the image signal when necessary.
When the radiation image detecting system of the present invention is provided with an image display means which displays an image on the basis of the standardized digital image signal and a standardization characteristic changing means for changing the characteristic of the standardization processing, the characteristic of the standardization processing can be corrected while observing the reproduced image when the characteristic is not acceptable.
When the radiation image detecting system is further provided with an image signal output means which, when the characteristic of the standardization processing is not changed within a predetermined time after an image is displayed by the image display means, automatically outputs the digital image signal on the basis of which the image is displayed, the operator has only to correct the characteristic only when the characteristic is not acceptable.
When the radiation image detecting system is provided with an alarm means which gives the alarm when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics, an accident where a digital image signal standardized according to an unacceptable characteristic is output can be avoided.
Further when the radiation image detecting system is provided with a correction information output means which, when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics, outputs correction information for correcting the radiographing conditions to conditions which can present a digital image signal the characteristic of the standardization processing for which may be one of the predetermined characteristics, the operator can correct the radiographing conditions referring to the correction information and can retake a radiation image according to the corrected radiographing conditions.
Similarly when the radiation image detecting system is provided with a radiographing condition correcting means which, when the characteristic of the standardization processing as determined by the standardization characteristic determining means is not one of a plurality of predetermined characteristics, changes the radiographing conditions on the basis of one of the predetermined characteristics to conditions which can present a digital image signal the characteristic of the standardization processing for which may be said one of the predetermined characteristics, a radiation image can be retaken without any trouble to the operator.